Implantable cardioverter defibrillators (ICD) are used to provide defibrillation shocks that correct ventricular fibrillation and other arrhythmias that do not respond to lower-energy therapies. The defibrillation shocks are generated by using a low voltage battery to charge a capacitor. The batteries are generally primary batteries. One drawback of primary batteries is the need to explant the device in order to replace the battery after it has discharged its energy. Secondary batteries that can be recharged through the skin may reduce the need to explant a defibrillator in order to change the battery.
Secondary batteries have not been adopted by defibrillator manufacturers due to their high levels of self-discharge and high levels of calendar fade. Self-discharge is a measure of the rate at which a battery discharges when stored unused. It is a temporary loss, and the capacity discharged can be regained by charging the battery again. Batteries with high levels of self-discharge must be recharged frequently even when the battery is not used. As a result, a patient having a defibrillator with one of these batteries would need to frequently recharge the battery and would risk having a defibrillation event when the battery does not have the required capacity.
Calendar fade measures the amount of rechargeability that is permanently lost over the life of the battery. For instance, a battery with a high levels of calendar fade shows a drop in the maximum capacity to which the battery can be charged with the passage of time. As a result, the frequency with which the battery must be recharged increases over the life of the battery. Further, high levels of calendar fade can result in the need to explant the ICD and replace the battery. For the above reasons, there is a need for a defibrillator having an extended life and reduced maintenance requirements.